A continuous power supply is critical to the success of many applications. Telecommunications systems are a prime example of this, as they are expected to function continuously in the event of a power outage. A typical telecommunication power system converts the AC grid power to 48 volts DC, and then uses this to charge storage batteries and supply the load. In the event of a disruption to the AC grid power, critical plant is supplied directly from the storage batteries. Depending on the load size and required backup time, large amounts of energy storage are often necessary. Lead acid batteries are traditionally used as the storage element due to their relatively low costs, high energy density, and reliability.
However there has been a recent trend towards the use of valve regulated lead acid (VRLA) batteries due to perceived savings from reduced maintenance and ventilation requirements. The chemistry of a VRLA battery is the same as a conventional flooded lead acid battery, but the physical construction of a VRLA battery has been optimised to allow the gases produced during overcharge to be recombined back into water. Addition of water to replace that lost through gas venting is therefore not required (or possible) with a VRLA battery. In situations where there is a reliable AC power supply, years can pass before the power supply might be interrupted. During this time the storage batteries must be maintained in a fully charged state. All lead acid batteries have a natural self discharge, so a float charge must be supplied in order to maintain the battery in a fully charged state. Constant voltage float charge is normally recommended, and may be specified as a function of the battery temperature.
Float charging has two primary goals:    1) Ensuring the battery remains fully charged indefinitely; and    2) Maximising the life of the battery by maintaining ageing effects at minimum levels.
To ensure that a cell remains fully charged, both electrodes within the cell must be sufficiently polarised (raised above their fully charged open circuit rest potential). To maximise the life of a VRLA cell, grid corrosion and gas loss (venting) must be minimised. The traditional failure mechanism of lead acid batteries is excessive corrosion of the positive grid. The grid forms a low resistance path within the electrode, allowing large currents to be drawn from it. Grid corrosion reduces the cross-sectional conductor area, which increases its resistance. Eventually this resistance rises to a point where the cell can no longer supply the necessary current at the required terminal voltage. At this point the cell is said to have reached the end of its life. Due to the potentials involved, possible grid corrosion can never be completely eliminated, but it can be optimised to ensure the lowest possible rate. It is commonly accepted that the rate of positive grid corrosion is a function of the polarisation on the positive electrode, and has a minimum rate occurring at polarisation slightly greater than the open circuit rest potential. While there is some debate on the actual voltage at which the grid corrosion minima occur, a window of acceptable grid corrosion generally occurs between 40 and 80 mV. The polarisation associated with minimum grid corrosion may vary with cell chemistry.
A typical fully charged open circuit rests voltage for a VRLA cell is 2.14 V. For such a cell a float voltage of 2.27 V may be recommended. At this float voltage, a total polarisation of 130 mV must exist. If, for example, the optimal positive electrode polarisation for minimum corrosion exists at 50 mV, the need of electrode must be in support the remaining polarisation of 80 mV. As both electrodes are raised above their open circuit potentials, the primary goal of the charge will also be satisfied, and the cell will be maintained in a fully charged state indefinitely.
Traditionally an optimal (recommended), float charge voltage is determined on sample cells in a laboratory with the aid of a reference electrode. This recommended float voltage is then applied (largely unchecked) to cells in service in the field. A further complication is that 2 Volt cells are connected in series to produce the desired system voltages (typically 24 or 48 Volts). A single supply is then used to charge the series connected cells. Small differences between cells (resulting from manufacturing variance) may produce a distribution of cell voltages, despite all cells receiving an identical float current due to the series connection.
There is an industry trend towards reducing battery maintenance, so longer life batteries are desirable. However it is becoming increasingly apparent with many “long life” VRLA batteries that either poor design, or poor quality manufacturing, results in the cells failing prematurely in the field. This is believed to be due to internal electrode balance problems, and in particular negative plate discharge. As grid corrosion is the traditional failure mechanism, an obvious way to improve the battery life is to reduce the rate of grid corrosion. This may be achieved by altering the grid alloy. However for balanced float charge operation, the current associated with corrosion of the positive grid must balance the current associated with (impurity related) hydrogen evolution at the negative electorate. If the grid corrosion rate is reduced and purity of the negative electrode is not appropriately increased, polarisation of the negative electrode must decrease to supply current for hydrogen evolution. If the current associated with the hydrogen evolution at the negative electrode is sufficiently large when compared to the current consumed through positive grid corrosion, the entire applied polarisation will be supported by the positive electrode. A gradual discharge of the negative electrode must result in order to supply the current required for hydrogen evolution. While this rate of negative electrode discharge is extremely low, the cumulative effects of months or years of float charge can be significant. Furthermore, as the applied polarisation is supported entirely by the positive electrode, increased rates of grid corrosion, gassing, and possible dryout must result.
Analysis and subsequent optimisation of the float charge relies heavily on knowledge of the polarisation distribution between the positive and negative electrodes within a cell. Conventionally, an optimal float charge is determined by the cell manufacturer and supplied virtually unmonitored to all cells of that type in field use.
However, due to the importance polarisation plays in float charge optimisation, a number of schemes have been published that use varying designs of reference electrodes in cells for float polarisation analysis and subsequent control purposes. Examples include U.S. Pat. No. 3,657,639 (Willihnganz), U.S. Pat. No. 4,935,688 (Mistry), and U.S. Pat. No. 6,137,266 (Chalasani). Without exception, all of these systems require cell modifications to facilitate the use of the reference electrode or reference cell. As VRLA cells basically function as a sealed unit, it is difficult to insert the reference electrode without disturbing the seal and modifying the cells' characteristics.